she made herself out to be the one who controls Mom’s money, the one who has all the say to what Mom does with her money,
Unless your mother has been legally declared incompetent, she has authority over her own assets. She can rescind your sister's power of attourney any time. I think she should. My lawyer's advice to me was that no one person should have sole power of attourney. Maybe she should give P/A to all her kids jointly. Then you all will be forced to work together, even if you hate each other's guts. That would also ensure that no sister could "block" another out.
When I had power of attourney for my parents it was for their convenience, so that I could get things done when they couldn't. I didn't presume to tell them what they could or couldn't do with their money. I occasionally asked about large transactions, but that was to ensure that someone wasn't defrauding them. If they chose to buy gifts or give money to their grandkids or my sisters, that wasn't my business. If I didn't agree with what they did I kept my mouth shut, because at the end of the day it was their money, and they were mentally competent.
About the hate, it takes time to get over that, but hate is a very negative emotion that will hurt you more than them. I'm no expert on how to do it. I'm still a work-in-progress.
W